Restructure access to SummarySlices

Currently f.summaryslices.slices[frameindex].ch0 returns an array with the representation of the summary rate for the ch0 of every DOM.

I propose that, in analogy with timeslices, access to the tree is restructured so that slices[frameindex][DOMID] is a numpy array with the rates for the 31 PMTs of the DOM.

A method to convert the rate to kHz would be also needed.